For if you are using a Flash-based uploader like Uploadify and Rails 3.

flash_session_cookie_middleware.rb

## Inside: app/middleware/flash_session_cookie_middleware.rb

require 'rack/utils'
 
class FlashSessionCookieMiddleware
  def initialize(app, session_key = '_session_id')
    @app = app
    @session_key = session_key
  end
 
  def call(env)
    if env['HTTP_USER_AGENT'] =~ /^(Adobe|Shockwave) Flash/
      params = ::Rack::Request.new(env).params
      env['HTTP_COOKIE'] = [ @session_key, params[@session_key] ].join('=').freeze unless params[@session_key].nil?
    end
    @app.call(env)
  end
end

session_store.rb

## Inside: config/initializers/session_store.rb

Rails.application.config.middleware.insert_before(
  ActionDispatch::Session::CookieStore,
  FlashSessionCookieMiddleware,
  Rails.application.config.session_options[:key]
)

The gem is a big win, given the number of people doing this and because most of the documentation is still dealing with Rails 2.x. Good work, just need to find more ways to get the word out so nobody else has to mess with it.